Antibodies that specifically bind MOPEG have been elicited in rabbits. Unlabelled MOPEG specifically displaces enzymatically synthesized 3H-MOPEG from these antibodies, and as little as 3 ng of MOPEG can be measured. None of the catecholamines or their metabolites, including MOPEG-SO4 and 3,4-dihydroxyphenylethylglycol, bind to the antibodies.